Summary of Work: These studies are determining the kinetics that describe the fate of selected PCB congeners following dermal, oral and iv exposure. Data from these studies are being used in the construction of kinetic models to permit extrapolation to this broader class of chemicals to predict human health risks associated with environmental exposure. Results obtained to date indicate that PCB bioavailability following oral exposure is minimally effected by structural characteristics of the congeners studied. Approximately 90% all PCBs studied are absorbed from the gastrointestinal tract. However, bioavailability following dermal exposure is highly dependent upon the degree and position of chlorination. The less chlorinated congeners are much more bioavailable than the more highly chlorinated congeners. Bioavailablity following dermal exposure decreases from approximately 90% for monochlorobiphenyls to less than 10% for hexachlorobiphenyls. Within a group, e.g., tetrachlorobiphenyls, bioavailability varies with the position of chlorination. Those congeners having chlorine atoms in the para position are less readily absorbed from the skin. Skin penetration constants correlate well with log Kow. Following absorption into the skin metabolism of PCBs by enzymes in skin greatly facilitates absorption into the systemic circulation. Metabolites formed in skin partition into the systemic circulation and are readily excreted. Thus, most of that portion of the dose of the more poorly metabolized higher chlorinated congeners absorbed into the skin was retained in skin two weeks after the dermal dose was removed. Data developed in this work have been used to construct a mathematical model that predicts the fate of a range of PCBs following dermal exposure. This model has been used to make preliminary predictions of the fate of all the components of a commercial PCB mixture.